There is a need in many applications to provide adjustable positioning of an object relative to the three orthogonal Cartesian axes (x, y, and z), while at the same time providing rigid constraint against tipping, tilting, or rotation about these axes. Example applications for this type of positioning capability include medical, instrumentation, and manufacturing applications.
There are basically two different design approaches used for precision positioning of an object or its supporting platform relative to three orthogonal Cartesian x, y, and z axes. In the more conventional approach, some type of “translation stage” is used as the mechanism for providing adjustable translational positioning along one axis while also providing rigid constraint against motion in every other mechanical degree of freedom. Where it is desirable to provide adjustable translational positioning along each of three independent axes, it is common practice to use three cascaded translation stages. While this approach achieves the intended result (that is, adjustable positioning in the three orthogonal translational degrees of freedom with constraint against rotational motion), it typically does so at the cost of complex, expensive, and redundant hardware. Each translation stage must provide constraint against rotation in three degrees of freedom and against translation in 2 degrees of freedom. This type of solution, then, solves the problem of constraining rotations three times, once in each translation stage. In this first design approach, the connections made to the object are “cascaded”.
In contrast, a configuration following the second design approach would make 6 connections directly to the body, where each connection constrains one degree of freedom. An example of this second approach would be the well known “Stewart Platform,” which contains 6 struts connected to the body in a pattern of 3 intersecting pairs. The Stewart platform can be made to provide adjustable constraint in up to 6 degrees of freedom by controlling the length of each of the struts. However, the Stewart platform has the limitation that strut length adjustments do not produce orthogonal motion of the platform in Cartesian coordinates. Position adjustments of the Stewart platform are highly “coupled,” so that it is not possible to change the length of one strut and achieve a pure motion in only one Cartesian degree of freedom. This is in contrast to the adjustments of the individual translation stages of the first design approach.
Another example of the second design approach employs orthogonally disposed flexible couplings within a positioning apparatus. For example, U.S. Pat. No. 6,757,985 (Hall) discloses an apparatus for positioning a work platform that employs multiple flexible shaft couplings, such as bellows couplings. These couplings are aligned along Cartesian x, y, and z axes and are connected between a base and a ball joint attached to the work platform. In contrast to the more conventional approach using cascaded translation stages, this second design approach uses a number n of individual connections, where each connection is intended to constrain 1 degree of freedom. For example, to allow a body 1 degree of freedom, 5 constraining connections would be made between the body and a reference base. In the U.S. Pat. No. 6,757,985 apparatus, each coupling provides a connection that constrains a single degree of freedom.
The U.S. Pat. No. 6,757,985 apparatus has some advantages because it provides the ability to achieve independent Cartesian axis motions without redundant hardware. Because of this, the U.S. Pat. No. 6,757,985 apparatus combines the best of both design approaches. However, the U.S. Pat. No. 6,757,985 apparatus can be further improved by using couplings with improved characteristics and by integrating simpler means for making translation adjustments. Among the characteristics of a coupling which would make it ideally suited for use in this type of apparatus are:                (i) High wind-up stiffness        (ii) Low stiffness in degrees of freedom other than wind-up        (iii) Simple, low cost construction        (iv) Extended range of motion along a central axis (z direction).        
Thus, it can be seen that there is a need for an apparatus that allows independently controlled, precision positioning of an object in three orthogonal axes, using moderately priced components that do not requiring costly machining or assembly, and that is capable of improved performance over conventional positioning systems.